1. Field of the Invention
The present invention relates to an apparatus and method for imaging an object using echoes of observation waves such as an ultrasonic image diagnosis, a weather observation radar, or the like.
2. Description of the Related Arts
An ultrasonic diagnostic device is widely spread as a device for observing a state of a structure, a movement, and the like of an organism tissue interior in non-invasion. The ultrasonic diagnostic device generically transmits ultrasonic waves into an organism tissue and receives echoes (reflected waves) reflected from the organism tissue interior, and implements a signal process with respect to this echo signal to extract various pieces of useful information, and images it to display. As typical displays, one of them is a B mode tomographic display in which ultrasonic beams are scanned to reconstruct the echo signal according to the scan pattern to display a two-dimensional tomogram, and is also a Doppler tomographic display in which a Doppler component of the echo signal is extracted to display a two-dimensional distribution of a speed of a moving body.
As is commonly known, ultrasonic waves are reflected on an interface between media of different acoustic impedances, thereby forming an echo signal. Accordingly, the most conspicuous echo signal is reflected from an interface between internal organs, an interface between the internal organ and its interior or a surrounding liquid, or the like, and a shape of the internal organ, etc. can be read from the B mode image, etc. Furthermore, there are many cases that the acoustic impedance is different between a normal region and a lesion region even in the same internal organ, and from this nature, it is possible to find out the lesion region from the B mode image, etc.
The organism tissue as an object of the ultrasonic diagnosis device has a finer structure than wavelength of ultrasonic waves for use in a diagnosis. Accordingly, not only a definite echo from the interface but also a scattered echo due to the fine structure of the internal organ and the other soft tissue interior are returned. The scattered echoes due to these fine structures interfere with each other, so that a so-called speckle pattern consequently appears in this ultrasonic image.
Conventionally, such the speckle pattern is considered to be an artifact, and some devices are taken so as to produce the speckle pattern as little as possible. Lately, it is possible to make very acute a transmission ultrasonic pulse (namely a wide band), and such the expansion of a pulse band is useful greatly for a reduction in the speckle.
It is difficult to find out the speckle because the speckle itself is an image which appears as a result of interference with the echo from each part of the fine structure, and the speckle is same with the echo from a boundary surface between the normal tissue and the lesion region in view of a signal. Namely, at a level of the signal process, it is possible to distinguish between the signal components of the speckle and the echo signal from the boundary surface. Accordingly, a judgment whether a noticeable part of the image is a part of a large structure of the boundary surface, etc. or the speckle is entrusted to one (viewpoint) of an observer who looks at it. In many cases, the judgment whether or not the speckle is delicate, and only the sufficiently experienced observer could judge it accurately.
In the above description, such a problem was designated that the speckle is captured as negative information, and the negative information cannot be distinguished in view of a signal from positive information of the boundary surface of the lesion region, etc., and there is a case where it is difficult to distinguish in view of images. On the other hand, if considering that the speckle is attributable to the fine structure of about wavelength of the tissue, it is considered that the speckle pattern is captured as the positive information reflecting the fine structure of the tissue, and is positively put to some use in the diagnosis.
However, even in this case, such a problem is confronted that the speckle cannot be distinguished from the echo of the boundary surface in view of the signal.
If the speckle part can be imaged by distinguishing it from the boundary surface of a large structure in any shape, the observer can come to recognize the speckle and a part of the large structure by distinguishing, but there did not conventionally exist an imaging apparatus or method proposed from such a viewpoint.
In the above description, the prior art and its problems were explained exemplifying an ultrasonic diagnostic field, but the same problem occurs in a weather observation radar, etc.
The present invention was conceived to solve such the problems, and in the apparatus and method for imaging an echo of observation waves, it is an object of the present invention to provide a technique for distinguishing a speckle pattern by a fine structure of a size of about wavelength of the observation waves from a large structure to image. Furthermore, it is an object of the present invention to extract characteristics of the fine structure.
In order to attain the above objects, this echo image forming apparatus according to the present invention comprises: a wave transmitter for transmitting an observation wave pulse of a wide band; a wave receiver for receiving an echo of the observation wave pulse transmitted to generate a reception signal; a filter for extracting signal components of a plurality of different frequencies predetermined from the reception signal obtained by the wave receiver; and a display device for imaging separately respectively the signal components of each frequency extracted by the filter and displaying images corresponding to resultant frequencies mutually relating to each other.
Herein, as the observation waves, various waves such as ultrasonic waves, electromagnetic waves, or the like can be used in correspondence with objects or targets of observation.
With this structure, the signal components of the plurality of frequencies predetermined are extracted from the reception signal of the wide band by use of the filter. The speckle patterns by the fine structure of about wavelength in correspondence with the corresponding frequency emerge respectively in each extracted frequency component. Conventionally, the expansion of bands of the transmission waves decreases influences of the speckle, but according to the present invention, components of a specified frequency are daringly fetched out from the reception signal of the wide band, thereby making prominent the speckle. However, when the signal of the extracted single frequency is merely imaged, as described already, it is impossible to distinguish the image part representing the boundary surface of the large structure from the speckle.
Then, according to the present invention, the signal components of the plurality of frequencies are extracted from the wide band reception signal, and imaging is performed in each of respective signal components, and they are displayed mutually relating to each other.
The speckle is generated due to interference between the scattered waves in the fine structure, and as is commonly known, the interference depends on a relationship between a wavelength and an interval of a wave source (in this case, an interval of the fine structure). Accordingly, if a frequency of a signal differs (namely, wavelength), the looking speckle differs. Namely, when the speckle is seen in the signal of a certain frequency, it is considered that the speckle part has the fine structure of the interval in correspondence with the frequency (wavelength), and this part does not generate interference by the observation waves of the frequency differing there from. Accordingly, it occurs that, if a speckle, it shows up in the image of the signal component of a certain frequency, and it does not show up in the image of the signal component of another frequency. On the contrary, the echo from the boundary surface shows up in the signal components of any frequencies.
Accordingly, if displaying respective images generated from the signal components of the plurality of frequencies relating to each other, it is possible to judge that the image emerging in all images is the boundary surface, and it is possible to judge that the image emerging only in any one image is a speckle of the fine structure. Furthermore, in the speckle part, the interval of the fine structure of the part can be assumed according to appearance in which frequency""s image. Namely, according to the present invention, in the fine structure, it is possible to obtain quantitative information which has not been obtained in the prior art.
There may be various methods for displaying the images in correspondence with each frequency relating to each other, and as a preferred embodiment, there is a method in which the images in correspondence with each frequency are assorted by coloring, respectively, and are displayed by overlaying. By such overlaying, the boundary part between the speckle part in correspondence with each frequency and the large structure can be confirmed at a glance. Furthermore, the interval of the fine structure in each part can be discriminated by assortment by coloring.
In one aspect of the invention, a system for setting the frequency extracted by a filter as three types and corresponding the signal components of each frequency to each color of the three primary colors of an output device (a display or a printer device) is preferred from several viewpoints. For example, in the part of the boundary surface, all levels of the signal components of three frequencies are increased, but on the contrary, in the part of the fine structure, only a level of the signal component of a frequency matching the interval of the structure is increased. Accordingly, in the output device of a color increasing mixture system such as a CRT display, etc., as all levels of the three primary colors (for example RGB) in the part of the boundary surface are increased, a display is whitish, and the part of the fine structure is displayed in a color close to the primary color. Accordingly, it becomes easy to distinguish the fine structure and the boundary surface in displaying. In particular, when displayed as the B mode tomographic image, it is possible to grasp at a glance which part of respective tomographic surfaces is the boundary surface and which part has which degree of fineness in the fine structure. Furthermore, when used as a signal of the primary color (for example, cyan, magenta, yellow) of the output device of a color decreasing mixture system such as a printer, etc., as the signal levels of all the primary colors are high in the part of the boundary surface, it is blackish, and the part of the fine structure is expressed in vivid coloring. Accordingly, it is possible to express with the part of the fine structure outstanding.
Furthermore, in another aspect of the invention, a system for performing brightness modulation in the images at intensity of the reception signal of an original wide band is considered. According to this, the display reflecting the fine structure according to the present invention is possible by overlaying on the common image display in the prior art. For example, when the B mode tomogaphic image is formed by this system, the speckle pattern having a color representing the fine structure is overlapped on the tomographic image (monochroic image) having a superior resolution, and it is easy to specify a location of each fine structure.
Furthermore, in yet another aspect of the invention, the echo image forming apparatus comprises: a wave transmitter for transmitting observation waves of a plurality of different frequencies from the substantially same position; a wave receiver for receiving an echo of the observation waves of each frequency of the transmitted waves to generate a reception signal of each of respective frequencies; and a display device for imaging separately respectively the reception signal of each of the respective frequencies to display an image corresponding to each resultant frequency mutually relating to each other.
In this aspect, the plurality of observation waves having different frequencies are transmitted and the reception signal of each frequency is imaged, and the images are displayed mutually relating to each other. According to such system also, the same result as in the above system for extracting the signal components of the plurality of frequencies from the echo of the transmission observation waves of the wide band to image can be obtained.
Incidentally, in the above respective aspects, the images are formed respectively from the signal of the plurality of different xe2x80x9cfrequencies,xe2x80x9d and here the xe2x80x9cfrequencyxe2x80x9d follows an ideal sense in a principle, and actually this xe2x80x9cfrequencyxe2x80x9d is a frequency band having a width to some degree. Accordingly, in case of the plurality of different frequencies, it is desired to understand that they designate the plurality of frequency bands which can be made a sharp distinction significantly.